Al-Qaeda actually gamed out this aspect of the American response and concluded (apparently correctly) that the US was ill-prepared for a multi-hijacking scenario. This wasn't rocket science... They basically looked at all available information on threat modeling the US had done, and noticed that we had constrained ourselves to a single hijacking event. They correctly surmised that intelligence infrastructure (including NORAD) would be off of their playbook if they hijacked multiple planes at once.

This led to disruption of the response. When flight 77 (Pentagon plane) was reported as having dropped off of the transponder radios, by the time NORAD was notified the information had become garbled in a game of telephone and they thought they were receiving reports on flight 11, which is one of the two that struck the Trade Center towers. They scrambled jets to New Jersey instead of onto any predicted flight path for flight 77.